Juxta-articular myxoma within the suprapatellar pouch masquerading as a ganglion cyst

This article presents the case of a 63-year-old man who noted painless swelling within the suprapatellar pouch, which he attributed to an effusion. Small-magnet, lower-extremity magnetic resonance imaging (MRI) and subsequent arthroscopy missed the lesion. On MRI with gadolinium contrast, the lesion was defined but misdiagnosed as a suprapatellar pouch ganglion cyst. Following resection of the 6 x 5 x 1.8-cm lesion, histology confirmed a lobular benign tumor with cystic elements recognized in the pathology and radiology literature as juxta-articular myxoma. Such lesions are characteristically multilobulated and contained within a capsular rim that enhances with gadolinium venous contrast. Otherwise, they appear hyperintense to fat on T2-weighted images and hypointense to muscle on T1-weighted images. This is an uncommonly encountered but known cystic lesion around the knee that is most often confused with ganglion cyst, synovial lipoma, lipoma arborescens, or pigmented or non-pigmented villonodular synovitis. Given its more cellular nature and thicker encapsulation, it should be able to be differentiated from ganglion cyst on MRI with a high index of suspicion, and should be recognized because of its high rate of recurrence. Unusual in this case was its location within the suprapatellar pouch, demonstrating the ease with which such lesions can be missed arthroscopically despite significant mass.